Schistosomiasis in antiquity.

INTODUCF1ON SCHISTOSOMES that infect domestic animals are of great economic importance, and may even cause death in cattle and sheep.' These parasites may also at times infest man, the most important of the several species being Schistosoma haematobium and Schistosoma mansoni. They are the subject of the present discussion. Both these parasites have a complicated, but similar, life cycle. Mature adult male and female schistosomes inhabit the small mesenteric or pelvic veins of man, the definitive host. After fertilization, the eggs are eventually extruded into the urinary tract or the rectum, causing haemorrhage from these sites. In general, infection by S. haematobium tends to localize in the genito-urinary system, whereas infection by S. mansoni is more likely to be found in the rectum. Their mature eggs are, therefore, found either in the urine or faeces, or in the walls of the bladder or rectum. Both eggs and schistosomes may, however, be in aberrant sites, such as the internal viscera or in the superficial tissues.2 On discharge from the body, the eggs hatch out in fresh water, and the free-swimming miracidia infest snails ofthe family Physidae or Bulinidae for S. haematobium, and of the family Planorbidae for S. mansoni infections. These multiply asexually within the visceral mass of the snail, the alternative host, and when conditions are suitable, free-swimming larval forms, cercariae, escape into the water, penetrate the skin of the definitive host, man, and become immature schistosomes. Eventually, after passing through the lungs, they migrate to the venules of the pelvis or mesentery, where they mature in situ and lay eggs intravascularly, thus repeating their life cycle. Natural infection by S. haematobium is restricted to primates, and only five cases occurring among monkeys and baboons have so far been discovered in East and West Africa;3 it appears to be primarily an intestinal disease in monkey hosts.4 On *P. B. Adamson, M.D., D.T.M. & H., Beckwith House Farm, Beckwith, Harrogate, North Yorkshire HG3 1QQ.